Vagina. A woman can have the perfect body, a wonderful personality, a beautiful heart, a pure soul, flawless character... but if your cock doesn't feel right in her vagina, as much as you love and respect her, your sex life with her will be very frustrating. If however she is 60 lbs overweight, nasty, evil, vile, rude, insufferable, ugly, yet if your cock absolutely rejoices inside her vagina... your sex life with her will be wonderful.
I wouldn't recommend being anything to the 2nd kind of woman but a living Sybian fucking machine. As to the first, she's a keeper. Just make sure she's ok with you banging the 2nd chick, too. But when you bring the topic up, I further suggest you hasten to add that she is free herself to fuck anyone else she wants to.